1. Field of the Invention
The present invention relates to a recording and reproducing medium and to an apparatus for recording and reproducing data in the medium by using a laser beam.
2. Description of the Related Art
In general, optical disks (e.g., a compact disk) have been widely used for reproducing data in music or moving picture fields, because of the large capacity and low price thereof. The optical disks have also been used for peripheral equipment such as a computer, etc. In this field, it is required that the optical disks record and reproduce data. The optical disks generally use a thermal recording method. In this method, a laser beam for recording and reproducing data, emitted from a light source such as a laser device is condensed at a minute spot of a recording and reproducing medium. The optical characteristics of the minute spot region of the recording and reproducing medium are changed when being heated, whereby data is recorded therein. In such a thermal recording method, since the efficiency at which the energy of the laser beam is converted into heat is not high, high-speed recording and reproducing are difficult to perform, requiring the light source for a laser beam with a high output.
In recent years, in order to overcome the above-mentioned problem, a reproducing method using a surface plasmon resonance phenomenon is proposed in Japanese Laid-Open Patent Publication No. 4-14620. In this method, a recording layer can be made thin, so that data is recorded using a light source for a laser beam with a relatively low output.
Hereinafter, a conventional example of a recording and reproducing system aiming at high-speed recording will be described with reference to FIG. 9.
A medium 4 is filled between a prism 103 and a metal layer 101. The medium 4 has a refractive index ns which is smaller than the refractive index np of the prism 103 and has a thickness of d1. In general, the medium 4 should satisfy np&gt;ns. The medium 4 is usually air (in this case, ns is about 1). As an incident light 106, a P-polarized light having a wavelength of .lambda. is incident upon the prism 103 at an incident angle .theta. larger than a critical angle .theta.c. When the incident light 106 is incident upon the prism 103 at a particular angle .theta.1, an evanescent wave generated under an interface (i.e., the bottom face of the prism 103) is resonated with the vibration of plasma present between the medium 4 and the metal layer 101, thereby causing the surface plasmon resonance phenomenon. Because of this, the reflectance of the incident light 106 is reduced. In general, it is desired that the thickness d1 is smaller than a wavelength .lambda. of the incident light 106.
FIG. 10 is a graph showing the relationship between the reflectance of the incident light 106 and the incident angle .theta. thereof, where .theta. is a variable. A solid line shows the change in reflectance of the incident light 106 with respect to the incident angle .theta.. As is understood from FIG. 10, the angle of the incident light 106 at which the surface plasmon resonance phenomenon is caused is .theta.1. When a dielectric thin film 102 having a thickness of d2 smaller than d1 is formed on the metal layer 101, the angle of the incident light 106 at which the surface plasmon resonance phenomenon is caused is .theta.2, and in this case, the characteristic curve of the reflectance is represented by a broken line in FIG. 10. In this way, the characteristic curve of the reflectance with respect to the incident angle .theta. in a portion where the dielectric thin film 102 is formed is different from that in a portion where the dielectric thin film 102 is not formed. If the surface of a recording and reproducing medium is scanned under the condition that the incident angle .theta. is fixed at .theta.1, the amount of reflected light in a portion where the dielectric thin film 102 is formed is different from that in a portion where the dielectric thin film 102 is not formed, whereby a reproduced signal A can be obtained.
On the other hand, during the recording, a laser beam is directly irradiated to the dielectric thin film 102 and the dielectric thin film 102 is melted or sublimed with the heat generated at a beam spot to form a pit therein. The dielectric thin film 102 can be formed of an organic monomolecular film or a layered structure including a plurality of films.
However, in the above-mentioned structure, recording data is performed by melting or subliming selected portions of the dielectric thin film 102, so that recording new data cannot be repeatedly performed. In addition, separate light sources are required, respectively for recording and reproducing. Moreover, it is difficult to improve the recording density, because the recording density is determined by the spot size of a laser beam.